The in vivo misincorporation of naturally occurring amino acids has not been measured in senescent tissue. As a result, error hypotheses of aging have yet to be convincingly evaluated. To measure amino acid misincorporation, it is proposed that the major milk proteins, the caseins, be used. Because bovine caseins lack certain amino acids and can be obtained in large quantities, sufficient amounts of mis-synthesized protein would be present for the isolation and characterization of those molecules which contain specific errors. By purification of the protein molecules with specific errors, the analysis of misincorporation will be accurate and unambiguous. The methods to be developed will have universal applicability to amino acid misincorporation research.